View source for Parazoa

{{Short description|Ancestral subkingdom of animals}}
{{automatic taxobox
| fossil_range = {{long fossil range|760|0|[[Tonian]]–Present}}
| image = SpongeColorCorrect.jpg
| image_caption = [[Sponge]]
| classification_status = obsolete as paraphyletic
| taxon = Parazoa
| authority = Grant & Todd, 1838
| subdivision_ranks = [[Taxon|Taxa]]
| subdivision = 
* '''Porifera''' ''sensu lato'' {{small|(Eumetazoa hypothesis)}}
**[[Porifera]] ''sensu stricto'' {{small|(modern sponges, Eumetazoa hypothesis)}}
***[[Silicarea]]
****[[Demospongiae]]
****[[Hexactinellida]]
***[[Calcarea]] ''s.l.''
****{{extinct}}[[Heteractinida]]
****[[Calcarea]]
***[[Homoscleromorpha]]
***[[Stromatoporoidea]]
**'''Sponges with cyath body plan'''
***{{extinct}}[[Cribricyatha]]?
***{{extinct}}[[Archeocyatha]]
***{{extinct}}[[Radiocyatha]]
***{{extinct}}''[[Proarchaeocyathus]]''
**{{extinct}}''[[Eocyathispongia]]''
**{{extinct}}''[[Olgunidae]]''
**{{extinct}}''[[Otavia]]''
*[[Ctenophora]]? {{small|([[Benthozoa|Myriazoa]] hypothesis)}}
*{{extinct}}"[[Vendobionta]]"?
[[List of ediacaran genera]] for more.
| synonyms = *Enantiozoa, Delage 1892
*[[Porifera]] ''s.l.''
}}

'''Parazoa''' ('''Parazoa''', gr. Παρα-, para, "next to", and ζωα, zoa, "animals") is an obsolete [[kingdom (biology)|subkingdom]] that is located at the base of the phylogenetic tree of the [[animal]] [[Kingdom (biology)|kingdom]] in opposition to the subkingdom [[Eumetazoa]]; they group together the most primitive forms, characterized by not having proper tissues or where, in any case, these tissues are only partially differentiated. It generally includes a single phylum, [[Porifera]], which lack [[muscles]], [[nerves]] and internal [[Organ (biology)|organ]]s, which in many cases resembles a cell colony rather than a multicellular organism itself. All other animals are eumetazoans and [[agnotozoa]]ns (Agnotozoans are possibly paraphyletic or even nonexistent in studies), which do have differentiated tissues.

==Porifera and Archaeocyatha==
[[Porifera]] and [[Archaeocyatha]] show similarities such as benthic and sessile habitat and the presence of pores, with differences such as the presence of internal walls and septa in Archaeocyatha. They have been considered separate phyla,<ref>Whittaker, R. H. (1969). New concepts of kingdoms or organisms. Science 163 (3863): 150–160.</ref> however, the consensus is growing that Archaeocyatha was in fact a type of sponge that can be classified into Porifera.<ref>Stephen M. Rowland 2001, [https://www.jstor.org/stable/1307076?seq=1#page_scan_tab_contents Archaeocyaths: A History of Phylogenetic Interpretation] Journal of Paleontology Vol. 75, No. 6, 75th Anniversary Issue (Nov., 2001), pp. 1065-1078</ref>

==Porifera and Placozoa==
Some authors include in Parazoa the poriferous or [[sponge]] [[phylum|phyla]] and [[Placozoa]] on the basis of shared primitive characteristics: Both are simple, show a lack of true tissues and organs, have both asexual and sexual [[reproduction]], and are invariably aquatic. As animals, they are a group that in various studies are at the base of the phylogenetic tree, albeit in a [[paraphyletic]] form. Of this group only surviving sponges, which belong to the phylum [[Porifera]], and ''[[Trichoplax]]'' in the phylum [[Placozoa]].

Parazoa do not show any body symmetry (they are asymmetric); all other groups of animals show some kind of symmetry. There are currently 5000 species, 150 of which are freshwater. The larvae are planktonic and the adults are sessile. The Parazoa–Eumetazoa division has been estimated to be 940 million years ago.<ref>{{cite journal|author1=Nikoh, Naruo |author2=Iwabe, Naoyuki |author3=Kuma, Kei-ichi |author4=Ohno, Mutsuhito |author5=Sugiyama, Tsutomu |author6=Watanabe, Yoko |author7=Yasui, Kinya |author8=Shi-cui, Zhang |author9=Hori, Katsuji |author10=Shimura, Yoshiro |author11=Miyata, Takashi |title=An Estimate of Divergence Time of Parazoa and Eumetazoa and That of Cephalochordata and Vertebrata by Aldolase and Triose Phosphate Isomerase Clocks|journal=Journal of Molecular Evolution |date=July 1997|volume=45|number=1|pages=97–106|doi=10.1007/PL00006208|pmid=9211740 |s2cid=23710913 }}</ref>

The Parazoa group is now considered paraphyletic.{{cn|date=August 2022}} When referenced, it is sometimes considered an equivalent to the Porifera.{{cn|date=August 2022}}

Some authors include the [[Placozoa]],<ref>"Reviews glossary". Retrieved 2009-03-20.</ref> a phylum long thought to consist of a single species, ''[[Trichoplax adhaerens]]'', in the division, but sometimes it is also placed in the Agnotozoa subkingdom.

==Phylogeny==
According to the most up-to-date phylogeny, Porifera should not have a direct relationship with Placozoa. In any case, placozoans are likely simplified "coelenterates" without common characteristics with sponges.<ref>{{Cite journal|last1=Laumer|first1=Christopher E.|last2=Fernández|first2=Rosa|last3=Lemer|first3=Sarah|last4=Combosch|first4=David|last5=Kocot|first5=Kevin M.|last6=Riesgo|first6=Ana|last7=Andrade|first7=Sónia C. S.|last8=Sterrer|first8=Wolfgang|last9=Sørensen|first9=Martin V.|last10=Giribet|first10=Gonzalo|title=Revisiting metazoan phylogeny with genomic sampling of all phyla|journal=Proceedings of the Royal Society B: Biological Sciences|language=en|volume=286|issue=1906|article-number=20190831|doi=10.1098/rspb.2019.0831|issn=0962-8452|pmc=6650721|date=10 July 2019|pmid=31288696}}</ref><ref>{{Cite journal |last1=Laumer |first1=Christopher E. |last2=Gruber-Vodicka |first2=Harald |last3=Hadfield |first3=Michael G. |last4=Pearse |first4=Vicki B. |last5=Riesgo |first5=Ana |last6=Marioni |first6=John C. |last7=Giribet |first7=Gonzalo |year=2018 |title=Support for a clade of Placozoa and Cnidaria in genes with minimal compositional bias |journal=eLife |volume=2018;7 |article-number=e36278 |doi=10.7554/eLife.36278 |pmid=30373720 |pmc=6277202 |doi-access=free }}</ref><ref>Roberto Feuda et al. 2017, [http://www.cell.com/current-biology/fulltext/S0960-9822(17)31453-7 Improved Modeling of Compositional Heterogeneity Supports Sponges as Sister to All Other Animals] Current Biology, Volume 27, Issue 24, p3864–3870.e4</ref>

{{Clade | style=font-size:100%;line-height:100%
|label1=[[Animalia]]
| 1={{Clade
  |1=[[Porifera]]
  |label2=[[Eumetazoa]]
  |2={{Clade
    |1=[[Ctenophora]]
       |label2=[[ParaHoxozoa]]
      |2={{Clade
      |1=[[Placozoa]]
  |2={{Clade
     |1=[[Cnidaria]]
     |2=[[Bilateria]]}}}}}}}}}}

==References==
{{reflist}}

==External links==
{{wikispecies|Parazoa}}
{{commonscat}}
{{Animalia}}
{{taxonbar|from=Q205716}}
[[Category:Parazoa]]
[[Category:Animals]]